Photoelectric device, mainly for geodetical and astronomical instruments



United States Patent PHOTOELECTRIC DEVICE, MAINLY FOR GEO- DETICAL ANDASTRONOMICAL INS'I'RUMEN'IS Kurt Steglich, Berlln-Spandan, Germany,assl'gnor to Askanla-Werke A.G., a corporation of Germany ApplicationOctober 1, 1956, Serial No. 613,283

Claimspriorlty, application Germany October 1, 1955 2 Claims. (Cl. 88-1)This invention relates to a device for objectively determining, ingeodetical and astronomical instruments and the like, the preciseposition of a light emitting or light reflecting target such as a star,a transit target or the like. The device may for instance replace thesubjective observations which can be made by a human eye and may provideinstead an electric eye or photoelectric device in order to enhance theprecision of locating or tracking the target; however, the device isimproved, according to one aspect of the invention, with regard to thefacility and accuracy of making observations of the same'target, at thesame time or successively, by the human eye as well as the electric eye.According to another aspect of the invention the device is improved withregard to observations made on the same target by a plurality ofelectric eyes. The improvement has been provided by a new arrangement ofa well-known optical element of spectrally selective reflectance andtransmissivity, which replaces a device wherein a total reflectoris'mechanically operated. Details will be clear from the completedescription, which follows.

In the drawing, Figure 1 is a schematic, central section, in plan view,of an embodiment of the invention. Figure 2 is a fragmentary plan viewof a modified embodiment. Figures 3 and 4 are fragmentary views of afirst and a second modification, respectively, of a detail shown inFigure 1, these views being taken along a line transverse to the planeofFigure 1.

Figure 1 shows the tube 1 of the sighting telescope of a theodolite, forinstance for first order triangulation. It may be assumed that thetheodolite shall be focussed on a socalled target point" of luminouscharacter with precision centering of the point" image in the imageplane of the instrument, for which purpose the telescope can be adjustedin a horizontal plane by small adjustments in either of the directionsof the arrows. The target itself is not shown, nor is it believed to benecessary to show the precision adjustment mechanism, the inventionbeing concerned with the determination of required adjustments, not withthe means for making them.

In the case of purely human observation subjective errors of adjustmentare caused by the scintillation of the target image, which scintillationis caused by the unavoidable movement of air masses, in all geodeticalor.

similar observations; and the magnitude of such scintillation and of theerrors caused thereby is significant in case of precision or highprecision work. According, to one feature of the invention, an electriceye can be arranged so as to be free of such error; and such electriceye can be provided with sufiicient sensitivity to perform observationswhich are beyond the capacity of the human eye, as to spectralcomposition, intensity etc.

In the image plane 2 of the telescope objective 3 there is arranged astationary optical element 4, 5 for splitting the target image into twolaterally juxtaposed halves. The preferred image splitter comprises, inaccordance with the applicant's copending application S. N. 536,912filed September 27, 1955. now abandoned, a rhombic 2,917,967 PatentedDec. 22, 1959 2 prism 4 and a forty-five degree prism 5 cementedthereto; the hypotenuse surface of prism 5 bemg tn contact with one sidesurface of the rhombic prism 4 and one part of this side surface havingthereon a reflector coating 6 which has a sharply defined edge 7,separating it from the non-mirrorized part of this side surface. Theedge 7 lies exactly in the center of theimage plane 2 constituting thesight of objective 3; in the drawing it may be visualized as a straightline normal to the plane of the paper. Thus the light from one half ofthe target area passes through the beam splitter without deflection,while that from the other half is reflected by coating 6; and this isfurther reflected by the opposite side surface 8 ofthe rhombic prism, sothat in effect, the image splitter furnishes two separate, parallelbeams belonging respectively to the two halves of the target area. It isassumed that the target area normally has homogeneous luminance; this isusually the case in extraterrestrial targets and can be enforced, ifnecessary, in terrestrial targets.

As further shown in said prior application the two separated light beamsare received in a single light sensitive receiver or photocell. Thisreceiver may for instance consist in an electron multiplier 9, theseveral electrodes of which lie below the single, curved electrode whichis shown; this representation of course being used for purelydiagrammatic purposes. (It may be well to remember at this point thatthe metallic layers used in such devices are filled with electrons, someof which are emitted by photon or photoelectron impact if the layer canbe penetrated by photons or. photoelectrons, which it can if the atomstructure of the metallic layer is suitable as for instance in the caseof potassium and caesium oxide and if, in addition, the wave length of xthe received light is small enough to provide the so called thresholdfrequency. In the photocells used in normal practice the thresholdfrequencies lie in the ultraviolet region of the spectrum or in theso-called blue color band or generally: outside a major portion, such asthe green and red band, of the visible spectrum; the green and red colorbands have little effect, by comparison. These facts are well known buthave not, thus far, been considered in connection with instruments ofthe present type.

The receiver [alternately receives one and the other partial light beam,by virtue of suitable means, for instance a swinging shutter 10 actuatedby electromagnet 11 to alternately out oh the two partial beams. Thephotocell output currents produced by the two separate and alternatingbeams are compared with one another by a symmetrical subassembly 12,which may be iden'-' tical with the subassembly 15 to 21 of said priorapplication. There is therefore shown the same electronic switch 18,time constant members 19, 20 and galvanometer 21 as in said priorapplication; the members 19, 20 being associated with an amplifier 22 ofthe voltages generated in cell 9, through a pair of electron tubes 23,

24. By means of the cooperation of said electronic target image, asinfluenced by scintillation, thereby pro viding the required data foradjustment in azimuth. In order to adjust the telescope l in thesecondcoordinate, here in altitude, visual means may be used. Such means havethus far been provided by means comprising a deflector mirror insertablein the ray trace of the objective 3 ahead of the beam splitter 4, 5; andsuch an interceptor has usually been combined with an ocular 14 Iincluding a suitable marker plate, thereby providing an optical system,additional to the receiver system 9, 12, for observing the target in away independent of and additional to the observation by said receiversystem.

Ilse mechanical insertion of the interceptor mirror has been one of theremaining drawbacks of instruments of this type. The mechanicalrequirements to be fulfilled by the inserting mechanism areextremclyhigh if precue coordination of the beam splitter edge 7 withthe corresponding mark on the ocular marker plate shall be preserved. Asubstantially rigidly inserted deflector is required at this point, inorder to insure full'prccision.

Ordinarily, when a deflector is rigidly inserted in asimilarposition,wherelighttransmissionisalsorequired,v

there arises the question whether the sacrifice of intensity ofreflected or transmitted light is not too great. Accord- ,ing to thepresent invention, however, such sacrifice or loss of light has beenThis follows from a further consideration of the aforementionedthreshold frequencies of the photoelectric means; a consideration whichbads to the use of an interference partial reflector 13 or colorselective mirror, rigidly inserted in the ray trace.

Thus an ultraviolet threshold multiplier 9 allows practically fullutilization of visible frequencies in the ocular, while a blue thresholdmultiplier allows full utilization of the red and green band in theocular. Other examples could obviously be formulated.

It will now be understood that the invention is not limited to thepractical application as described and illustrated up to this point. Thedeflector mounting may be non-rigid, in cases where it is both possibleand worth while to provide suflicient accuracy of insertion,mechanically; the resulting arrangement is still benefited by theadvantage of the particular light frequency distribution, as indicatedabove, whenever optical observations are required simultaneously withthe photoelectrical observation mirror or partial reflector element 13may be rigidly mounted but may be combined, not with an ocular but witha device located and oriented like the ocular 14 but constructed likethe unit 16, 12 constituting the abovedescribed photo-analyzer; thissecond unit 16A, 12A (Figure 2) being used for automatic altitudeadjustment when the first unit 16, 12 is used for azimuth adjustment asmentioned. The two photo-tubes 9 will then be selected to providedifferent frequency thresholds and the interference mirror 13 will beconstructed to transmit target image, outer parts of the bundle ofimaging rays, 60

between the objective 3 and the beam splitter 4, but directly adjacentthe latter. Preferred examples of such means are furnished by anadjustable iris diaphragm (Figure 4) or a similarly arranged multiplediaphragm (Figure 3) with openings of different diameter. BecauseAccording to another modification the interferenceoftheinclinedsrrangementotthebeamsplittersurfacecarryingtheedge7andbecauseofthepresenceotglsssofprisms4and5infrontofthisedge,thediaphragmcannotbeplacedintheplaneoftheimageitself;however 5ithasbeenfoundsutflcient, forthepurposesstated,toplacethisdiaphragmdirectlyinfrontoftheseprimso thatist forms an aperturecontrol member for the objectrve The adjustable iris diaphragm ormultiple diaphragm may be conventional by itself. For instance Figure 4shows the iris diaphragm as comprising the usual series of swingablevanes 25, I, 27, 2a, which may be pivoted in the usual housing 29 andmay be adjusted conjointly, bythemualmechanismnotshowmunderthecontroloflianadj'ustmentlever. SimilarlyFigureZishowsa multiple diaphragm plate31 having a series of apertures 32, 33, 34, 35 formed therein and whichplate can beshiftedacrosstheaxisoftheinstrumentinwellknownways,withtheaidofguidingandmanipulatingparts,not shown.

I- claim:

1. In a telescope for observing and tracking a luminous object withoutdisturbance by scintillation and with minimum loss of light: anobjective; a first and a secend optical system, both adapted to receivelight from the object through the objective; a color selective partialreflector stationarily mounted between the objectiveontheonehandandthetwoopticalsystemsontheotherhand,withapartialreflectorsurfaceinclinedto optical axis of theobjective, for selectively passinghghtoffrequenciesinonemajorportionofthevisibleandulu'avioletspectrumtothefirstopticalsystemfor said observing, whilepassing light of other frequenciesofthespectrumtothesecondopticalsystem;and,for

said tracking and as part of at least one of said optical systems, aphotometric analyzer, having threshold sensitivity for light frequenciesin the respective portion of the spectrum, a stationary image splitterbetween the partial reflector and the analyzer, with an image splittingedge intersecting the optical axis of the objective, and

shutter means between the image splitter and the, analyaer foralternately shutting olf image portions split by the image splitter.

2.Inatelescopeasdescribedinclaimlthefeature that the partial reflectorselectively reflects at least the red and green band while selectivelytransmitting at least the ultraviolet band.

RelereneesCltedinthefileofthispatent UNITED STATES PATENTS 1,322,780 FmNov. 25, 1919 1,410,110 Siedentopf Oct. 16, 1923 2,302,554 KingsburyNov. 11, 1942 2,462,925 Varian Mar. 1, 1949 2,513,367 Scott July 4, 1950FOREIGN PATENTS 352,035 Great Britain June 22, 1931 OTHER REFERENCESWhitford et al.: Photoelectric Guiding of Astronomi' cal Telescopes,"Review of Scientific Instruments, vol 8, pages 78-82, March, 1937.

